1. Field of the Invention
The present invention relates to an engine variable valve timing system and particularly to an engine variable valve timing system equipped with a hydraulic variable intake phase mechanism and a hydraulic variable exhaust phase mechanism.
2. Description of the Related Art
Recent automotive engines are equipped with variable valve timing systems that vary the timing at which the intake and exhaust valves open and close. These variable valve timing systems typically have variable phase mechanisms that vary the phases of the camshafts with respect to the crankshaft. Such variable phase mechanisms have conventionally been disposed on the ends of the intake camshaft and the exhaust camshaft. Variable phase mechanisms comprise a sprocket linked by chain to the crankshaft, a housing formed as a unit with the sprocket and a rotor formed as a unit with a camshaft enclosed within the housing, so that an advancing hydraulic pressure chamber and a retarding hydraulic pressure chamber are formed by means of the housing and the rotor. Thus, by controlling the supply or discharge of hydraulic pressure (advancing hydraulic pressure or retarding hydraulic pressure) to or from these hydraulic pressure chambers using a control valve or the like, for example, it is possible to change the phases of the camshafts with respect to the crankshaft and as a result it is possible to vary the timing of the opening and closing of the intake and exhaust valves.
In this case, as taught by Japanese Patent Unexamined Publication (Kokai) No. JP-A-2001-50102, portions of the hydraulic lines that connect these hydraulic control valves to the advancing hydraulic pressure chamber and the retarding hydraulic pressure chamber constitute annular grooves provided on the bearing surface of the cam cap which supports the camshafts. Moreover, the advancing hydraulic pressure and the retarding hydraulic pressure are supplied from the annular grooves via hydraulic lines passing through the interior of the camshafts and on to the advancing hydraulic pressure chamber and the retarding hydraulic pressure chamber.
However, if the overlap period during which the intake and exhaust valves are both open is large such as during idling, for example, deleterious effects such as the engine speed becoming unstable may occur, so in this case, it is customary to attempt to shorten the overlap (i.e, advance the open/close timing of the exhaust valve and/or retarding the open/close timing of the intake valve) and thus suppress the suckback of exhaust from the exhaust line. On the other hand, during low and medium loads as during low-speed driving, it is customary to attempt to increase the overlap (i.e., retard the open/close timing of the exhaust valve and/or advancing the open/close timing of the intake valve) and thus improve fuel economy and the like.
However, there is the problem of the response lag of hydraulic fluid from when the signals for advance/retard control are output until the advancing hydraulic pressure and the retarding hydraulic pressure are supplied to or discharged from the advancing hydraulic pressure chamber or the retarding hydraulic pressure chamber, and the valve timing is actually advanced or retarded. In particular, when the accelerator pedal is released from a low-load to medium-load state wherein the overlap is large, it is necessary to reduce the overlap, but due to the response lag in the supply or discharge of hydraulic pressure, the state with a large overlap is maintained despite the idling state. If this happens, the engine speed becomes unstable as described above, possibly leading to a stall. More specifically, delay in the supply of retarding hydraulic pressure to the retarding hydraulic pressure chamber of the variable intake phase mechanism or delay in the supply of advancing hydraulic pressure to the advancing hydraulic pressure chamber of the variable exhaust phase mechanism may result in delayed response in advance/retard control, so when changing the overlap from large to small, it will not become small immediately.
In addition, when the previous engine halt occurred suddenly, as when going from a high-load state without adequately passing through the idling state, or in the case of a stall or the like, because of the hydraulic fluid response lag described previously, it is possible that the intake and exhaust camshafts may not have returned adequately to the side of narrow overlap (the exhaust camshaft on the advanced side, the intake camshaft on the retarded side). Even in this case, it is sufficient for the hydraulic pressure to rise at the time of the next engine start and for the camshafts to return promptly to the side of narrowing the overlap, but when the engine halts the hydraulic pumps are also halted and supply no hydraulic pressure, so while the engine is halted the hydraulic fluid is bled from the hydraulic pressure chambers and the hydraulic lines connecting these hydraulic pressure chambers to the hydraulic pressure control valves described above, so one cannot expect the hydraulic pressure to rise immediately upon the next engine start. As a result, the engine is started in the state in which the open/close timing of the intake/exhaust valves is not appropriate (the overlap is not sufficiently narrow), so there is a problem in that the engine ignition and starting performance become poor.
In particular, a return spring that constantly presses the intake and exhaust valves toward the closed side is incorporated into the engine valve train mechanism. This return spring becomes resistance to camshaft rotation and as a result, the camshaft is subject to a reaction force in the retarding direction when the valve is open. Moreover, this reaction force in the retarding direction causes the intake camshaft to be pressed in the direction of narrowing the overlap, while the exhaust camshaft is conversely pressed in the direction of enlarging the overlap. Thus, while the intake camshaft is naturally or easily returned in the retarding direction that narrows the overlap while the engine is halted or at the time of an engine start, the exhaust camshaft is not easily returned in the advancing direction that narrows the overlap.
There are further problems that may occur in variable phase mechanisms on which a lock mechanism is mounted. Specifically, this lock mechanism is defined to be one where, when the camshaft and the rotor reach the position at which the overlap is narrowest (the most advanced position on the exhaust side and most retarded position on the intake side), a lock pin provided on the rotor is pressed toward the sprocket side and engages an indentation provided on this sprocket side, so that the rotor and sprocket are linked as a unit.
At this time, this lock pin may be constituted such that it may be knocked out from this indentation when hydraulic pressure is supplied to a special hydraulic pressure chamber, and the hydraulic pressure used to knock out this lock pin is typically the hydraulic pressure supplied to the exhaust-side advancing hydraulic line and advancing hydraulic pressure chamber, namely the hydraulic pressure used for advancing. This lock mechanism is intended to keep the camshaft at the narrowest overlap position while the engine is halted (in other words, at the most preferable position for starting the engine), so it is fundamentally unnecessary while the engine is running. Moreover, immediately after the engine is started, the hydraulic pressure is controlled so as to make the overlap narrower on the exhaust side, so immediately after the engine is started, a situation occurs in which the advancing hydraulic pressure rises first while the retarding hydraulic pressure has not yet risen. Accordingly, in order to quickly unlock the lock mechanism which is no longer necessary once the engine is started, the advancing hydraulic pressure for exhaust which rises immediately after the engine is started is used to unlock the lock mechanism described above.
However, the hydraulic pumps are also halted when the engine is halted, so the hydraulic fluid that had flowed in the hydraulic lines drains downward and air enters these hydraulic lines. Moreover, when the engine is next started, on the exhaust side, the hydraulic pressure control valves exert control of the hydraulic pressure so that the exhaust camshaft moves toward the advancing side (so that the overlap becomes narrower). Specifically, on the exhaust side, hydraulic fluid is first supplied to the empty advancing hydraulic line and the advancing hydraulic pressure chamber, but hydraulic fluid is not yet supplied to the equally empty retarding hydraulic line and the retarding hydraulic pressure chamber. Moreover, at this time, the air within this hydraulic line is first pushed out by the hydraulic pressure supplied to the advancing hydraulic line, and there is a possibility that this air may knock the lock pin of this lock mechanism out from the indentation. Furthermore, at that point in time, the exhaust-side advancing hydraulic pressure (hydraulic line) and the intake-side retarding hydraulic pressure have not yet reached the advancing hydraulic pressure chamber and the retarding hydraulic pressure chamber, so ultimately problems occur wherein the rotor and camshaft positions fluctuate unstably, abnormal sounds are caused by shimmying in the direction of rotation between the rotor formed as a unit with the camshaft and the casing formed as a unit with the sprockets which form the hydraulic pressure chambers, or the position of the rotor and camshaft shifts from the advanced-side position, making the rotation during idling become unstable.